Method for the production of a dental moulded part

ABSTRACT

In a method for the production of a dental moulded part, in which there is produced a coquille ( 10 ) having a hollow space ( 11 ) the form of which corresponds in substance to the form of a model ( 2 ) of the moulded part, and in which by filling of the coquille hollow space ( 11 ) with a hardenable material a casting ( 20 ) is produced and hardened, the model ( 2 ) is provided with an offset ( 2   a,    2   b,    2   c ) which is taken into account in the production of the coquille ( 2 ), wherein after the hardening of the casting ( 20 ) this is worked by material removal to produce the dental moulded part.

The present invention relates to a method for the production of a dentalmoulded part in accordance with the preamble of claim 1 or of claim 20.In particular the present invention relates to a method for theproduction of moulded parts of noble metal alloys or non-iron metalalloys which are difficult to work.

In dental technology tooth prosthetic provisions are produced of themost varied materials, depending upon indication, aestheticrequirements, health consciousness and financial situation of thepatient. As a result of new production technologies such as laserwelding, galvano-technics and not lastly through the advance of dentalCAD/CAM-systems, today there can be put to use, alongside the classicalnoble metal casting alloys, also new biocompatible materials in the formof semi-finished materials such as titanium, glass ceramics, highperformance ceramics or plastics.

The proportion of classical noble metal casting alloys, in theproduction of fixed or combined (so-called attachments) toothprostheses, is as before very high. This is the case because noble metalalloys have proved over decades to be very dependable in the mouth ofthe patient, tolerant in the case of constructional configuration anddesign faults and, not least, manifest a high biocompatibility. Afurther advantage of noble metal alloys further consists in that forthese moulded parts a proven range of tested and easily workable veneerceramics are available.

However, in the case of production of noble metal tooth prostheses withdental CAD/CAM systems against these advantages there stand the highstocking costs for the necessary semi-finished items and theconsiderable costs for the separation of the machining waste noblemetal, whereby it is to be taken into consideration that at the presenttime the level of machining waste lies above 90%. Consequently, to datemost attempts at a production of noble metal tooth prostheses by meansof CAD/CAM systems in dental or practice laboratories have failed forcost reasons.

In the production of tooth prosthetic parts of NIM (non-iron metal)alloy semi-finished products with the aid of CAD/CAM systems, theproblem of the difficult machinability of the Co—Cr—Mo ceramic alloysstands in the foreground. The savings though the employment ofeconomical NIM alloys are cancelled out by the high wear of expensivemilling machines and the long working times, which lead to high machinehour costs.

One proposal for avoiding the above-mentioned problems in the productionof tooth prostheses of noble metal or NIM alloys is so-called lasersintering. Such equipment requires, however, a huge investment of thelevel of several hundred thousand Euros, which cannot as a rule becommitted by a dental or practice laboratory. Further, with thelayer-wise build up of the parts physical limits apply to the precision,so that in turn long, complicated, mechanical and manual finishing isnecessary. The production of the fine metal powder necessary for thesintering process is, beyond this, technologically very complicated andparticularly in the case of noble metals not economic. Further, thedental technician must pass on the basis of his prosthetic work, i.e.pass it on to external workshops, so that waiting times arise and thedental technician again looses a substantial component of his financialchain of production.

A method for the production of dental moulded parts of noble metalalloys, which at least partly uses the advantages of a CAD/CAM system,is known from U.S. Pat. No. 5,224,049. Here, after a model of themoulded part to be formed has been produced, there is selected from aseries of predetermined mould forms (so-called coquilles), that coquillethe structure of which comes closest to the moulded part to be produced.If there are still deviations from the moulded part to be produced, thecoquille is worked for so long until the hollow space corresponds to themoulded part to be produced. Then, this is filled with material, forexample a noble metal alloy, which can be hardened.

By means of the method known from U.S. Pat. No. 5,224,049 the outlay forthe production of a dental moulded part is thus reduced; it has howeverbeen found that this method has certain limits with regard to itsprecision. In particular more complicated surface structures of thedental moulded part can be realized with the necessary precision onlywith difficulty.

The present invention thus is based on the object of indicating apossibility of producing also dental moulded parts with complicatedstructures in as rationalized a manner as possible. In particular thereshould be the possibility of being able to use the advantages CAD/CAMtechnology as comprehensively as possible.

The object is achieved by means of a method for the production of adental moulded part in accordance with claim 1 and by means of a methodin accordance with claim 20. The subclaims relate to advantageousfurther developments of the method in accordance with the invention.

In accordance with the present invention, the method for the productionof a dental advantage consists in substance of the following four steps:

a) production of a model of the moulded part to be formed,

b) production of a coquille, having a hollow space the form of whichcorresponds in substance to the form of the model,

c) production of a casting by filling of the coquille hollow space witha material which can be withstood and

d) allowing the casting to harden.

In accordance with a first insight according to the invention, the modelproduced in step a) is provided with an offset, which is taken intoaccount in the production of the coquille, whereby after the hardeningof the casting this is worked by material removal in order finally toproduce the dental moulded part. The thickness of the offset may therebydepend both on the form of the moulded part to be produced and also onthe hardenable material, and the offset may cover the model in part oralso completely.

The present invention links or supplements the advantages of classicalCAD/CAM technology with classical casting technology in a modified formand in this way makes possible a highly precise, partly automated andeconomical production of the tooth prosthetic workpieces of noble metalalloys or Co—Cr—Mo alloys in a dental or practice laboratory. Incomparison to the method known from U.S. Pat. No. 5,224,049, theproduction of the coquille is simplified, since through the employmentof the offset more complex structures in the surface of the moulded partto be produced can be compensated, so that the coquille can be workedrelatively quickly and simply. In comparison to the known method,although there is then still subsequently needed a working of thehardened casting, the advantage is however provided that with this inthe end the desired moulded part can be produced with a very highprecision. Beyond this, in accordance with the method in accordance withthe invention, also more complicated surface structures can be createdfor the moulded part, which otherwise could be realized only withdifficulty or not at all.

In order to be able to fully use the advantages of CAD/CAM systems, inaccordance with a particularly preferred variant of the method accordingto the invention it is provided that the coquille is produced by meansof fully automatic material removing working, in particular by means ofmilling, turning, boring, and/or grinding of at least one coquilleblank. Here, a material removal program taking into account the desiredcontours of the coquille gives control commands for a material removalmachine, wherein the program is produced in particular also the basis ofa three-dimensional CAD reconstruction of the model provided with theoffset. Further it can be provided that upon production of the coquilleat the same time at least one inlet channel is worked into the blank,via which then subsequently the hardenable material is filled.

Corresponding to a second insight of the present invention it isprovided that after the hardening of the casting this is, for finishingworking, still embedded—i.e. together with the coquille, placed in amould receiver and worked there. Preferably there is involved the samematerial removal machine which was already employed previously for theproduction of the coquille. Since in this case an exact repositioning ofthe still embedded casting in the tool machine is made possible, a highprecision of the subsequent working of the moulded part is ensured.

A particular advantage is further provided with this second inventiveinsight in that the coquille material can serve at the same time asmillable or grindable support or retaining material, so that also theworking of very thin and per se fragile castings is possible. After apartial milling away of the coquille material it can then be provided toagain embed behind the casting with another support material and thenwork the remainder of the casting occlusally.

Overall, there is thus provided by means of the present invention thepossibility of producing dental moulded parts of noble metal alloys orNIM alloys in a very effective manner, whereby at the same time aprecision as great as possible is ensured in the production of themoulded part.

Below, the invention will be described in more detail with reference tothe accompanying drawings. Thereby there is shown:

FIGS. 1 to 10 the various steps of a preferred exemplary embodiment ofthe method in accordance with the invention and

FIG. 11 a tabular overview of the method steps of the classical methodfor the production of a dental casting and of the method in accordancewith the invention.

The method in accordance with the invention for the production of dentalmoulded parts represents, in comparison to the classical method for theproduction of gold castings, a clear step forward, since many of theworking steps normally to be carried out can now be carried outautomatically or by machine. For clarification of the advantages inaccordance with the invention, the classical method for the productionof an individual gold cap will be briefly summarized below.

Thereby, after conclusion of the preparatory operations, initially afoil cap is deep drawn, which is then adapted to the previously workedtooth stump. After insulation of the tooth stump the foil cap is fixedand then a wax model of the crown to be made is produced. This wax modelis now lacquered and then the casting prepared, for which purpose firstthe muffle is prepared, a bedding mass slurry produced and the model ofthe wax crown finely embedded. After completion of the embedding, themuffle is waxed out and pre-warmed and finally poured with the liquidnoble metal alloy. After hardening of the casting, this is bedded outand the casting blank finish cleaned and the inlet channel separated.

Then, the cast piece produced in this manner can be worked with ahandpiece until it assumes the form of the desired crown cap. This capis then, for the purpose of surface treating, blasted and cleaned. Aftera cleaning of the tooth stump, the cap can then be put in place in thefinal operation.

The method known to date is summarized in tabular form in FIG. 11. Ascan be taken from the above summary, a great part of the tasks to beundertaken in the classical method are to be carried out manually,wherein some of the working steps take up a long period of time. Inparticular the modelling of the crown in wax and the working of thecasting with the handpiece take up relatively much time. In contrast tothis, with the aid of the present invention, the production method canbe significantly speeded up and automated, as explained below.

The beginning of the method in accordance with the invention therebycorresponds to the classical production method, i.e. within the scope ofpreparatory operations the tooth is first worked and then there isproduced with appropriate monitoring of the working documentation, andthe model, margins etc., the tooth stump illustrated in FIG. 1, providedin general with the reference sign 1, or a positive model of the same.The stump 1, produced by means of classical impressing and the like isthen three-dimensionally optically measured and for this purpose anappropriate measurement device put to use, as is generally known fromCAD/CAM systems.

As soon as the data regarding the three dimensional structure of thetooth stump is completely available, there can then be effected, PCcontrolled, an automatic surface reconstruction, which finally leads tothe construction of the crown cap. With the aid of classical CAD-3Dtechnologies, here the crown cap is largely automatically generated, onthe so-called NURBS (non-uniform rational B splines) surface; that is onthe stump geometry described with the aid of a mathematically complexmethod, and—so far as it is necessary—manually slightly corrected.Through this there is provided a three dimensional model in digital formof the cap provided in FIG. 2 with the reference sign 2, which withregard to its structure corresponds exactly to the moulded part to beproduced.

A significant step of the method in accordance with the first inventiveinsight is illustrated in FIG. 3, in which the cap 2 is provided withinthe scope of the three dimensional modelling or CAD reconstruction withan offset 2 a, that is an additionally applied layer on the cavityand/or occlusal side. This offset 2 a may depend on the one hand uponthe structure on the moulded part to be produced and on the other handupon the type of material, and typically lies in the range from ca. 0.5mm. It may partially or completely cover the model 2.

In accordance with the illustration in FIG. 4 this offset 2 a can thenbe additionally modelled, i.e. provided with supplementary layers 2 band 2 c, so that overall there is provided an offset form, generallygiven the reference sign 3, which approximately corresponds to the formof the moulded part be produced, but—partially or on all sides—issurrounded with an additional layer. The supplementary modelling of theoffset 2 a can be employed in particular to form a relatively simplesurface structure for the offset form 3, through which the subsequentproduction of the coquille for carrying out the casting method isfacilitated.

In a next step a coquille is thus then to be produced, as it isillustrated in FIG. 5 and provided with the reference sign 10, whereinthe coquille 10 is to have an hollow space 11 which corresponds to thestructure of the moulded part provided with the—if appropriate,modified—offset.

The production of the coquille is effected, correspondingly to theillustrated exemplary embodiment, fully automatically, wherein initiallya NC program for the production of the coquille is generated with theaid of a CAM module and taking into account the model geometry. The CAMmodule thereby generates automatically the mould division and thesuitable milling program in order to form the hollow space 11.

The production of the coquille is illustrated in FIGS. 6 a and 6 b,wherein for this purpose initially a first coquille blank 12, consistingof a suitable material 13, is placed in a mounting bridge of a materialremoving machine, for example a milling, turning, boring or grindingmachine, and worked for material removal with an appropriate tool 5, inorder to form the lower outer surface 14 of the hollow space. A secondcoquille blank 15, illustrated in FIG. 6 b, is in turn so worked withthe aid of the tool 5 that the upper bounding surface 7 of the hollowspace is provided. Depending on the geometry of the coquille, one or twocoquille blanks are thus worked, preferably five-axis milled, so thatfinally in the assembled condition a hollow space is formed thestructure of with corresponds to the offset form 3 illustrated in FIG.4. During the carrying out of the milling procedure, at the same timethere may be automatically generated an NC program for the later workingof the casting, which is necessary in order to attain the desired crowncap geometry with a suitable cavity and the desired occlusal side.

It is important that in the working of the coquille blanks 12 or 15appropriate inlet channels 21—illustrated in FIG. 7—can already beprovided, which will later be used to fill the hollow space 11 with theliquid alloy. As soon as the coquille or the coquille halves have thenbeen appropriately worked they can be taken out with the mounting bridgeand—insofar as two coquille blanks are involved—assembled together andput in place in the casting apparatus.

The coquille formed in this way is then filled with the noble metalalloy or the Co—Cr—Mo alloy, whereby as a result of the special,millable coquille material, a pre-heating before the castingcorresponding to the manner of procedure known to date is no longernecessary. As soon as the casting provided through this has sufficientlyhardened, the coquille 10—consisting of the two halves 12 and 15, can beagain placed in the mounting bridge 19 of the milling machine (FIG. 7),whereby due to the possibilities for precise repositioning, a precisionas great as possible is made possible in the later working.

The final working of the casting 20 is now effected in turn with the aidof a material removing tool 6, wherein corresponding to the illustrationin FIG. 8 initially the casting 20 is worked from the upper side, untilfinally a surface 25 arises which in its dimensions and in its structurecorresponds to the desired crown cap. In the working cycles “trimming”and “fine trimming” the offset is removed, with the slightest materialloss, so that a precisely sized cavity is generated. It is significantthat here the outer side of the casting 20 is supported by the remainingcoquille material 13 or 16 so that also very fine castings can beworked, without there being the danger that these are thereby damaged,in particular break apart.

Before a final working of the occlusal side the cavity side is againembedded with a bedding material 22, for example an appropriate millingwax or plastic, and thus supported, so that the mould part is safelymounted and supported also during the final working process illustratedin FIG. 10. In this process, the occlusal side is then milled, untilfinally also this side has a surface structure 26 which corresponds tothe desired crown structure.

After conclusion of the milling process, the worked casting 20 isremoved from the machine and released from the bedding wax or plastic,which e.g. may be effected by means of heating with the aid of a hot airblower. In rare cases final slight correction is necessary with the aidof a dental motor handpiece in order to work again the inner side of thecavity.

The crown cap provided in this way is then briefly blasted inside andoutside at the veneer surface in a blasting apparatus, whereafter, afterappropriate removal of contaminant particles and the like, with cleaningof the tooth stump, the cap is put in place and the final operations canbe carried out.

The described work procedure makes clear that in contrast to theclassical dental casting method the primary activities of the dentaltechnician are redirected to the CAD/CAM work station and -to the CNCcontrolled, preferably 5-axis, milling machine. The great part of thecomplex manual activities are thus avoided, as is also shown by thecomparison in the table of FIG. 11. Here it is clear that many workingsteps are transferred to the PC or the machine and in particular mayalso run automatically. The remaining manual tasks, in contrast, aremainly such that they can be carried out very quickly.

A further advantage is provided in that the millable coquille need not,as was to date usual, be pre-warmed in a controlled thermal process overan extended time, and transferred in the hot condition into the castingapparatus. Since the coquille material is well millable or grindablewithout great tool wear, after the coquille casting it can at the sametime serve as mounting of the cast piece in the mounting bridge. Herethere is provided as an advantage not only that a support of the castingduring the finishing working is effected. At the same time it is ensuredthat an extremely precise repositioning is made possible. Due to theseadvantages one can work with a very thin offset and expensive noblemetal can be saved. In the employment of Co—Cr—Mo alloys, in contrast,due to the slight volume removal, tool wear is reduced, which in turnmakes possible a production which covers its costs.

1. A method for the production of a dental moulded part, having thefollowing steps: (a) production of a model of the moulded part to beformed, (b) production of a coquille having a hollow space the form ofwhich corresponds in substance to the form of the model, (c) productionof a casting by filling of the coquille hollow space with a hardenablematerial and (d) allowing the casting to harden wherein, the modelproduced in step a) is provided with an offset which is taken intoaccount in the production of the coquille so that after the hardening ofthe casting the model may be worked by material removal to produce thedental moulded part.
 2. The method according to claim 1, wherein, atleast a cavity side of the model is completely provided with an offset.3. The method according to claim 1, wherein, at least an occlusal sideof the model is completely provided with an offset.
 4. The methodaccording to claim 1, wherein, the entire surface of the model isprovided with an offset.
 5. The method according to claim 1, wherein thethickness of the offset depends upon the form of the mouled part to beproduced.
 6. The method according to claim 1, wherein the thickness ofthe offset depends upon the hardenable material.
 7. The method accordingto claim 1, wherein the model of the moulded part to be formed isproduced on the basis of an optical measurement of a tooth stump onwhich the moulded part is to be arranged.
 8. The method according toclaim 7, wherein the model with the offset is produced with the aid of athree-dimensional CAD reconstruction, wherein data obtained in theoptical measurement of the tooth stump is taken into account.
 9. Themethod according to claim 1, wherein the coquille is produced by workingof at least one coquille blank with a material removing machine.
 10. Themethod according to claim 9, wherein the working of the at least onecoquille blank is effected fully automatically, by means of milling,turning, boring and/or grinding.
 11. The method according to claim 10,wherein a material removal program, taking into account the desiredcontours of the coquille and of the coquille hollow space, containscontrol commands for the material removing machine.
 12. The methodaccording to claim 10, wherein the material removal program is producedon the basis of the three-dimensional CAD reconstruction of the modelprovided with the offset.
 13. The method according to claim 9, whereinupon production of the coquille at least one inlet channel is workedinto the coquille blank.
 14. The method according to claim 1, whereinafter the hardening, the casting is placed together with the coquille ina mould receiver of a material removing machine.
 15. The methodaccording to claim 14, wherein for the working of the coquille blank andof the casting the same machine is employed, wherein the coquille isagain put in place in the tool receiver exactly repositioned.
 16. Themethod according to claim 14 wherein in the working of the casting thecoquille material serves as support or mounting material.
 17. The methodaccording to claim 16, wherein the working of the casting is effected inat least two steps, and in each step a certain region of the casting isworked, wherein the previously worked region of the casting issurrounded with a milling wax or a plastic bedding mass.
 18. The methodaccording to claim 14, wherein the working of the casting is effectedfully automatically by milling, turning, boring and/or grinding.
 19. Themethod according to claim 18, wherein a material removal program takinginto consideration the contours of the casting and of the moulded partto be produced contains control commands for the material removingmachine.
 20. A method for the production of a dental moulded part,having the following steps: a) production of a model of the moulded partto be formed, b) production of a coquille having a hollow space the formof which corresponds in substance to the form of the model, c)production of a casting by filling of the coquille hollow space with ahardenable material and d) allowing the casting to harden, wherein afterthe hardening, the casting is worked in a material removing fashion, inorder to produce the moulded part, and wherein for this purpose thecasting, not bedded out, is put in place together with the coquille in atool receiver of a material removing machine.
 21. The method accordingto claim 20, wherein in the working of the casting the coquille materialserves as support or mounting material.
 22. The method according toclaim 21, wherein the working of the casting is effected in at least twosteps, and in each case a certain region of the casting is worked,wherein the previously worked region of the casting is surrounded with amilling wax or a plastic bedding mass.
 23. The method according to claim20, wherein the working of the casting is effected fully automaticallyby milling, turning, boring and/or grinding.
 24. The method according toclaim 1, wherein the moulded part is formed of a noble metal alloy. 25.The method according to claim 1, wherein the moulded part is formed of anon-iron metal alloy.
 26. The method of claim 1, wherein the mouldedpart is formed of a Co—Cr—Mo alloy.